This invention relates to antennas, and more particularly to a dual polarization antenna specifically adapted to provide better communications in a helicopter environment.
High frequency (HF) radio systems (radios that operate between 2 and 30 mhz) have been used on helicopters and fixed wing aircraft for many years. The non-line-of-sight (NLOS) capabilities of the HF band are well known. It is also known that some HF propagation modes are extremely frequency dependant, requiring that the radio operator have an intuitive knowledge of the usable frequencies for a particular time of day. Often the radio operator must place calls on several frequencies before clear communication can be established. At times, communications may not be possible. This level of operator control renders the traditional HF radio system unusable in the combat helicopter environment. Most combat helicopters are therefore currently equipped with line-of-sight (LOS) radios only. However, many missions require them to employ nap of the earth (NOE) flight profiles in which current radio systems are not effective.
High frequency automatic link establishment (ALE) radio systems, such as the HF-9000 radio system manufactured by Rockwell/Collins, Inc., automate the operation of the traditional HF radio system, and render possible the employment of HF radio systems in combat helicopters. In such a system, the pilot establishes communication by placing a call to the desired party. This is easily accomplished by selecting the address of the party to call and keying the microphone. Operator knowledge of the proper frequency to use is not necessary for radio operation. The radio system chooses the best frequency for communication and automatically establishes a link with the desired contact. The pilot is notified when the link is established and can at that point communicate as with any other radio system. It is this automated capability that makes an HF radio system a practical solution for combat helicopters flying NOE NLOS flight profiles. The optimum frequency selection is based on the analysis of prior transmissions from the desired contact. If the database of prior transmissions is unavailable, the system automatically attempts communications on all available frequencies until communications success is achieved.
The theory behind HF propagation has been well understood for many years. In general, there are four primary propagation paths supported by the HF band that can allow NLOS NOE communications. The most commonly used propagation path in the HF band is skywave. Skywave propagation, by using ionospheric reflections or ionospheric scattering, can allow communications ranges exceeding several thousand miles. Near Vertical Incident Skywave (NVIS) is essentially high critical angle skywave propagation and can support communications to 300 km. Lastly, the ground wave propagation surface wave component can support communications to 40 or 50 km and the line-of-sight component can support communications to several hundred km depending on aircraft height. FIG. 1 illustrates the various HF propagation paths. Each of these primary propagation paths have variables involved that can drastically affect communications range and include multiple secondary propagation effects that at times are useful for communications.
The past approach employed in severe non-line-of-sight environments was to rely on NVIS propagation to reflect electromagnetic waves off of the ionosphere. The antennas used for this function were generally horizontal in nature in that they were mounted on helicopters along the tail boom. This horizontal orientation was necessary to couple the energy reflected off of the ionosphere into the antenna. The problem with NVIS propagation is reliability. The reliability is a function of the ionospheric electron density which in turn is a function of the amount of solar radiation from the sun. As a result, the NVIS communications reliability is often poor at night when the sun has set.
What is needed, therefore, is an antenna system which complements NVIS propagation with ground wave propagation to increase the communications reliability in an aircraft severe non-line-of-sight environment.